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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Coral reef | 5/13 | https://en.wikipedia.org/wiki/Coral_reef | reference | science, encyclopedia | 2026-05-05T07:34:41.179437+00:00 | kb-cron |
The Great Barrier Reef—largest, comprising over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometers (1,600 mi) off Queensland, Australia The Mesoamerican Barrier Reef System—second largest, stretching 1,000 kilometers (620 mi) from Isla Contoy at the tip of the Yucatán Peninsula down to the Bay Islands of Honduras The New Caledonia Barrier Reef—second longest double barrier reef, covering 1,500 kilometers (930 mi) The Andros, Bahamas Barrier Reef—third largest, following the east coast of Andros Island, Bahamas, between Andros and Nassau The Red Sea—includes 6,000-year-old fringing reefs located along a 2,000 km (1,240 mi) coastline The Florida Reef Tract—largest continental US reef and the third-largest coral barrier reef, extends from Soldier Key, located in Biscayne Bay, to the Dry Tortugas in the Gulf of Mexico Blake Plateau has the world's largest known deep-water coral reef, comprising a 6.4 million-acre reef that stretches from Miami to Charleston, S. C. Its discovery was announced in January 2024. Pulley Ridge—deepest photosynthetic coral reef, Florida Numerous reefs around the Maldives The Philippines coral reef area, the second-largest in Southeast Asia, is estimated at 26,000 square kilometres. They are populated by over 900 reef fish species and 400 scleractinian coral species, 12 of which are endemic. The Raja Ampat Islands in Indonesia's Southwest Papua province offer the highest known marine diversity. Bermuda is known for its northernmost coral reef system, located at 32.4°N 64.8°W / 32.4; -64.8. The presence of coral reefs at this high latitude is due to the proximity of the Gulf Stream. Bermuda coral species represent a subset of those found in the greater Caribbean. The world's northernmost individual coral reef is located in the Finlayson Channel, in the inside passage of British Columbia, Canada. The world's southernmost coral reef is at Lord Howe Island, in the Pacific Ocean off the east coast of Australia.
== Coral ==
When alive, corals are colonies of small animals embedded in calcium carbonate shells. Coral heads consist of accumulations of individual animals called polyps, arranged in diverse shapes. Polyps are usually tiny, but they can range in size from a pinhead to 12 inches (30 cm) across. Reef-building or hermatypic corals live only in the photic zone (above 70 m), the depth to which sufficient sunlight penetrates the water.
=== Zooxanthellae ===
Coral polyps do not photosynthesize, but have a symbiotic relationship with microscopic algae (dinoflagellates) of the genus Symbiodinium, commonly referred to as zooxanthellae. These organisms live within the polyps' tissues and provide organic nutrients that nourish the polyp in the form of glucose, glycerol, and amino acids. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Without their symbionts, coral growth would be too slow to form significant reef structures. Corals get up to 90% of their nutrients from their symbionts. In return, as an example of mutualism, the corals shelter the zooxanthellae, averaging one million for every cubic centimetre of coral, and provide a constant supply of the carbon dioxide they need for photosynthesis.
The varying pigments in different species of zooxanthellae give them an overall brown or golden-brown appearance and give brown corals their colors. Other pigments, such as reds, blues, and greens, are produced by colored proteins in coral animals. Coral that loses a significant fraction of its zooxanthellae becomes white (or sometimes pastel shades in corals that are pigmented with their own proteins) and is said to be bleached, a condition which, unless corrected, can kill the coral. There are eight clades of Symbiodinium phylotypes. Most research has been conducted on clades A–D. Each clade contributes both benefits and less compatible attributes to the survival of its coral hosts. Each photosynthetic organism has a specific level of sensitivity to photodamage to compounds needed for survival, such as proteins. Rates of regeneration and replication determine the organism's ability to survive. Phylotype A is found more in the shallow waters. It can produce mycosporine-like amino acids that are UV-resistant, using a derivative of glycerin to absorb UV radiation, thereby allowing them to better adapt to warmer water temperatures. In the event of UV or thermal damage, if and when repair occurs, it will increase the likelihood of survival of the host and symbiont. This leads to the idea that, evolutionarily, clade A is more UV resistant and thermally resistant than the other clades. Clades B and C are found more frequently in deeper water, which may explain their higher vulnerability to increased temperatures. Terrestrial plants that receive less sunlight because they are found in the undergrowth are analogous to clades B, C, and D. Since clades B through D are found at deeper depths, they require an elevated light absorption rate to be able to synthesize as much energy. With elevated absorption rates at UV wavelengths, these phylotypes are more prone to coral bleaching than the shallow clade A. Clade D has been observed to be high temperature-tolerant, and has a higher rate of survival than clades B and C during modern bleaching events.
=== Skeleton ===
Reefs grow as polyps and other organisms deposit calcium carbonate, the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral head's top upwards and outwards. Waves, grazing fish (such as parrotfish), sea urchins, sponges and other forces and organisms act as bioeroders, breaking down coral skeletons into fragments that settle into spaces in the reef structure or form sandy bottoms in associated reef lagoons. Typical shapes for coral species are named by their resemblance to terrestrial objects such as wrinkled brains, cabbages, table tops, antlers, wire strands, and pillars. These shapes can depend on the life history of the coral, like light exposure and wave action, and events such as breakages.
=== Reproduction ===